ORIGINAL: rick_reno
I'm from the Idaho Panhandle - there is almost no way you can heat up one of those bags during rifle season out here. It's either cold, or it's cold and cloudy. They just don't heat up that well.
They don't work well when temps drop much below 60 degrees , but you can always use them as a reservoir and heat water over the campfire . Showering outdoors during very cold temps is usually a bad idea anyway .